1. Field of the Invention
The present invention relates to a light source capable of emitting light over a wide wavelength range, and, more particularly, to a wideband light source using an erbium-doped fiber.
2. Description of the Related Art
A wavelength division multiplexed-passive optical network (WDM-PON) includes a central office for providing communication services via a plurality of downstream optical signals having wavelengths different from each other; hereinafter, referred to as “downstream optical signals”, a plurality of subscriber units for receiving downstream optical signals from the central office and outputting upstream optical signals to the central office, and at least one remote node for linking the central office to the subscriber units.
The central office uses a distributed feedback laser or a multi-wavelength laser as a light source for generating the downstream optical signals. Recently, an optical transmitter that generates wavelength-locked downstream optical signals has been suggested as a suitable light source.
The suggested optical transmitter includes a Fabry-Perot laser for generating wavelength-locked downstream optical signals and a wideband light source for outputting spontaneous emission light, including inherent light of wavelengths different from each other, to induce the Fabry-Perot laser into a wavelength-locked mode.
As such a wideband light source, a white light source capable of generating inherent spontaneous emission light is suggested. However, instead of the white light source having a low spontaneous emission output, an erbium-doped fiber amplifier is generally used as a wideband light source.
FIG. 1 shows the construction of a conventional wideband light source including an erbium-doped fiber amplifier. As shown, the conventional wideband light source includes a first amplifying medium 130 for generating and amplifying a C-band spontaneous emission light, a second amplifying medium 150 for generating an L-band spontaneous emission light, a first isolator 140, first and second optical couplers 121 and 161, a second isolator 170, first and second pump light source 122 and 162, and a reflector 110.
The first pump light source 122 generates a first pump light for pumping the first amplifying medium 130. The second pump light source 162 generates a second pump light for pumping the second amplifying medium 150. First and second pump light sources 122 and 162 may use, for example, a semiconductor laser.
The first optical coupler 121 is positioned between the reflector 110 and a first end 130a of the first amplifying medium 130 and is connected to the first pump light source 122 so as to transmit the first pump light to the first amplifying medium 130 and the C-band spontaneous emission light inputted from the first amplifying medium 130 to the reflector 110.
The first isolator 140 is connected to a second end 130b of the first amplifying medium 130 and a first end 150a of the second amplifying medium 150 to transmit the C-band spontaneous emission light inputted from the second end 130b of the first amplifying medium 130 to the first end 150a of the second amplifying medium 150. Also, the first isolator 140 prevents the L-band and C-band spontaneous emission lights from being inputted to the first amplifying medium 130 from the second amplifying medium 150.
The reflector 110 reflects the C-band spontaneous emission light inputted from the first optical coupler 121 back to the first optical coupler 121. The first optical coupler 121 inputs the C-band spontaneous emission light reflected by the reflector 110 to the first end 130a of the first amplifying medium 130.
The second isolator 170 outputs the C-band and L-band spontaneous emission lights inputted from the second optical coupler 161 through an output terminal and prevents external light from being inputted into the wideband light source.
The second optical coupler 161 is positioned between the second end 150b of the second amplifying medium 150 and the second isolator 170 so as to transmit the C-band and L-band spontaneous emission lights inputted from the second end 150b of the second amplifying medium 150 to the second isolator 170 and the second pump light inputted from the second pump light source 162 to the second amplifying medium 150.
U.S. Pat. No. 6,507,429 granted to Gaelle Ales et al. for an invention entitled “Article Comprising a High Power/Broad Spectrum Superfluorescent Fiber Radiation Source” discloses the conventional wideband light source as shown in FIG. 1.
However, the conventional wideband light source having the first isolator arranged between the first and second amplifying media produces lower amplification efficiency and does not utilize the backward C-band spontaneous emission light generated from the second amplifying medium. Furthermore, if the pump light sources are controlled in an attempt to control the outputs from the first and second amplifying media, the outputs will be influenced by each other. Therefore, it will not be easy to control the L-band and C-band spontaneous emission outputs.